Conventional MEMs mirrors used in optical switches, such as the one disclosed in U.S. Pat. No. 6,535,319 issued Mar. 18, 2003 to Buzzetta et al, for redirecting beams of light to one of a plurality of output ports, include an electro-statically controlled mirror pivotable about a single axis. Tilting MEMs mirrors, such as the ones disclosed in U.S. Pat. No. 6,491,404 issued Dec. 10, 2002 in the name of Edward Hill, and U.S. Patent Publication No. 2003/0052569, published Mar. 20, 2003 in the name of Dhuler et al, which are incorporated herein by reference, comprise a mirror pivotable about a central longitudinal axis. The MEMs mirror device 1, disclosed in the aforementioned Hill patent, is illustrated in FIG. 1, and includes a rectangular planar surface 2 pivotally mounted by torsional hinges 4 and 5 to anchor posts 7 and 8, respectively, above a substrate 9. The torsional hinges may take the form of serpentine hinges, which are disclosed in U.S. Pat. No. 6,327,855 issued Dec. 11, 2001 in the name of Hill et al, and in U.S. Patent Publication No. 2002/0126455 published Sep. 12, 2002 in the name of Robert Wood, which are incorporated herein by reference. In order to position conventional MEMs mirror devices in close proximity, i.e. with a high fill factor (fill factor=width/pitch), they must be positioned with their axes of rotation parallel to each other. Unfortunately, this mirror construction restraint greatly restricts other design choices that have to be made in building the overall switch.
When using a conventional MEMs arrangement, the mirror 1 positioned on the planar surface 2 can be rotated through positive and negative angles, e.g. ±2°, by attracting one side 10a or the other side 10b of the planar surface 2 towards the substrate 6. Unfortunately, when the device is switched between ports at the extremes of the devices rotational path, the intermediate ports receive light for fractions of a millisecond as the mirror 1 sweeps the optical beam past these ports, thereby causing undesirable optical transient or dynamic cross-talk.
One solution to the problem of dynamic cross-talk is to initially or simultaneously rotate the mirror about a second axis, thereby avoiding the intermediate ports. An example of a MEMs mirror device pivotable about two axes is illustrated in FIG. 2, and includes a mirror platform 11 pivotally mounted by a first pair of torsion springs 12 and 13 to an external gimbal ring 14, which is in turn pivotally mounted to a substrate 16 by a second pair of torsion springs 17 and 18. Examples of external gimbal devices are disclosed in U.S. Pat. No. 6,529,652 issued Mar. 4, 2003 to Brenner, and U.S. Pat. No. 6,454,421 issued Sep. 24, 2002 to Yu et al. Unfortunately, an external gimbal ring greatly limits the number of mirrors that can be arranged in a given area and the relative proximity thereof, i.e. the fill factor. Moreover, the external gimbal ring may cause unwanted reflections from light reflecting off the support frame 13, 14.
Another proposed solution to the problem uses high fill factor mirrors, such as the ones disclosed in U.S. Pat. No. 6,533,947 issued Mar. 18, 2003 to Nasiri et al, which include hinges hidden beneath the mirror platform. Unfortunately, these types of mirror devices require costly multi-step fabrication processes, which increase costs and result in low yields.
The solution to overcome the shortcomings of the prior art proposed by the inventors of the parent application listed above is to provide a high fill factor MEMs mirror device that can pivot about the same axis as an adjacent mirror. In a preferred embodiment the MEMs mirror device is relatively easy to fabricate, with an internal gimbal ring and applicable in high fill factor applications.
Typically MEMs devices are independently actuated by an electrode mounted on the substrate below the tilting platform for inducing a voltage-controlled rotation via an electrostatic torque. In open loop configuration, the angular position of the platforms depend non-linearly on the voltage applied by the electrodes, i.e. as the applied voltage is increased linearly, the incremental change in angular platform position is greater as the voltage increases. Accordingly, there is a maximum voltage, i.e. an angular platform position, at which the platform angular position becomes unstable and will uncontrollably tilt until hitting part of the lower structure. This maximum voltage sets the range of angular motion that the platform can travel. The instability in the platform's angular position is a result of the distance between the platform and the electrode (the hot electrode) decreasing more rapidly at the outer free ends of the platform than at the inner sections, nearer the pivot axis θy. As a result, the force per unit length along the platform increases more rapidly at the outer free ends of the platform than the inner sections.
An object of the present invention is to overcome the shortcomings of the prior art by increasing the range of angular motion of a MEMs device's platform by reducing the field strength, i.e. the force per unit area, that is sensed at the outer free ends of the platform.